As the problem of carbon dioxide emission into the atmosphere and energy problems have been actualized, biomass-derived chemical products represented by biodegradable polymer materials such as lactic acid and biofuels such as ethanol have attracted stronger attention as products with sustainability and life cycle assessment (LCA) capability. These biodegradable polymer materials and biofuels are generally produced as fermentation products from microorganisms using as a fermentation feedstock glucose, which is a hexose, purified from edible biomass such as maize. However, use of edible biomass may cause a rise in its price because of competition with food, resulting in an unstable supply of the feedstock. In view of this, attempts are being made to use sugars derived from non-edible biomass such as rice straw as a fermentation feedstock for microorganisms (see WO 2010/067785).
When a sugar derived from non-edible biomass is used as a fermentation feedstock, cellulose, hemicellulose and the like contained in the non-edible biomass are decomposed into sugars by a saccharifying enzyme. In that process, not only hexoses such as glucose, but also pentoses such as xylose are obtained, and as a consequence a mixed sugar of hexose and pentose is used as a fermentation feedstock if a sugar derived from non-edible biomass is used as a fermentation feedstock for a microorganism (see WO '785).
As a fermentation method in which a sugar derived from non-edible biomass, which is a mixed sugar of hexose and pentose, is used as a fermentation feedstock for a microorganism, continuous fermentation may be employed, but the fermentation yield actually achieved by continuous fermentation has not been studied (see WO '785). On the other hand, as known in the art, the culture medium is continuously used for the fermentation in the case of continuous fermentation using a mixed sugar of hexose and pentose as a fermentation feedstock, and therefore the fermentation yield in continuous fermentation is much lower than in batch fermentation since the microorganism continuously undergoes catabolite repression unlike batch fermentation (see Do Yun Kim, Seong Chun Yim, Pyung Cheon Lee, Woo Gi Lee, Sang Yup Lee, Ho Nam Chang, Batch and continuous fermentation of succinic acid from wood hydrolysate by Mannheimia succiniciproducens MBEL55E, Enzyme and Microbial Technology, 35, (2004), 648-653). Thus, according to the common technical knowledge, it has been thought that a microorganism that does not undergo catabolite repression has to be used in the fermentation to improve the fermentation efficiency of continuous fermentation using a mixed sugar of hexose and pentose as a fermentation feedstock for a microorganism.
Many microorganisms that undergo catabolite repression are known as microorganisms capable of fermentation production of biodegradable polymer materials and biofuels. On the other hand, it is known that continuous fermentation using a mixed sugar of hexose and pentose as a fermentation feedstock for a microorganism results in a remarkably decreased fermentation yield due to catabolite repression. In view of this, there is a need to improve the fermentation yield in continuous fermentation using a mixed sugar of hexose and pentose as a fermentation feedstock for a microorganism that undergoes catabolite repression.